1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing minute metallic spheres suitably used for forming a ball bump on an electrode of a semiconductor device, a printed circuit board, or the like.
2. Description of the Related Art
As methods of manufacturing minute metallic spheres within low-melting temparature, there are atomization method and a method of obtaining metallic spheres by dipping a metallic piece made into a predetermined volume in advance, in a heated liquid.
Besides, in the method described in Japanese Patent Application Laid-open No. 4-74801, a molten metal is extruded from very fine particles in a liquid heated to a temperature more than the melting point of the metal.
Although a large quantity of metallic particles can be obtained for a short time by the atomization method, it is difficult to make the shapes of the particles uniform in sphere and into an aimed size, and the yield becomes very bad. By the method of dropping a metallic piece in the heated liquid, it can be made into a substantially complete sphere by surface tension. But, for making the sizes of metallic pieces to be dropped, uniform in advance, there are some required processes of press-punching a plate made very thin by rolling or the like, or of making into a fine line with a die or the like and accurately cutting with a cutter or the like.
Besides, in that described in the Japanese Patent Application Laid-open No. 4-74801, the interior of a vertical tube is filled with a natural oil or the like, and a zone or a region having a temperature range more than the melting point of a metal is provided by a heater attached to the upper portion of the vertical tube. And, a low-melting alloy supply tube to which a fine grating is attached in that region, is stood such that the fine grating is at the lower position. A lump of a low-melting alloy is inserted in the low-melting alloy supply tube and molten, and an inert gas is fed through the upper portion of the low-melting alloy supply tube. Molten alloy is extruded from the fine grating by the pressure of the gas and made into particles, and they become spherical shape by passing through the temperature inclination of the vertical tube.
However, in the method described in this official gazette, the relation between the grating size and the pressurizing force, or the like, is unknown. In any case, the shape of a particle is made by the surface tension in a delicate balance relation to such a factor. In this method, so-called duplex grain can not be avoided.